1. Field of the Invention
This invention relates to dynamic dielectric analysis as a sensing methodology applied to real-time process monitoring for thermoplastic filament winding.
2. Background Art
Filament winding is a process where fibers are placed on a rotating mandrel in a predetermined path to form a given shape or part. By combining the fibers with a resin binder, a finished part with desirable mechanical properties results. Filament winding is primarily used to form products such as pipes, vessels, or tanks which are subject to internal pressure. Other applications, such as high strength, low weight control rods or other slender structural elements subject to torsion, are commonly filament wound.
Current filament winding techniques frequently use thermosetting resins which require post-curing procedures in an autoclave or oven. The advent of high strength thermoplastic resins has allowed the process to be completed at the time of winding since temperature and pressure can be applied simultaneously during the winding process. This process is called "in-situ" consolidation of the resin and fiber. Thermoplastic tape pre-impregnated with reinforcing fibers (prepreg tape) along its length is the usual product form used as input for an in-situ consolidation winding process.
An ability to consolidate the finished product during the winding process offers the advantage of eliminating the need for post-curing or autoclaving. The removal of autoclaving from the process provides strong economic incentives to apply in-situ consolidation to as many processes as possible. Additional advantages of the in-situ consolidation process over the traditional thermoset filament winding include the ability of making wound parts with low winding angles and thin cross sections.
Even with the above mentioned advantages, in-situ consolidation has yet to be widely accepted by the industry because current in-situ consolidated parts tend to have a higher void content than autoclaved parts. Void content refers to the number and volume of trapped air bubbles or other foreign objects in the resin of the finished product. Void content is normally referred to as a percentage of the overall volume of the part. Autoclaved parts can have void contents as low as 1% while in-situ consolidated parts fall in the range of 5-10%.
Current void content measurement for filament wound parts are completed off-line by ultrasonic attenuation, X-radiography, Fokker bond testing, optical holographic stress wave interferometry or thermography. However, none of these methods of measurement is currently applied in real time nor would they be suitable for application to a control process. This lack of suitability results from difficulties in implementation into current winding equipment, the speed at which measurements are made, or possible structural damage to the part itself.
Therefore, it is desirable to provide real-time monitoring of thermoplastic filament winding to ensure proper in-situ consolidation, especially considering the strong economic advantages. Industrial applications of thermoplastic filament winding with in-situ consolidation have been limited thus far by the void content of wound parts resulting from the variation of resin consolidation at the filament lay down point.